Lead-based paint was commonly used for residential, government and commercial buildings constructed prior to 1978. Damaged or aged lead-based paint coatings are sources of hazardous lead dust. Lead dust may remain on the damaged coating surfaces or become an airborne particulate hazard as the peeling paint spalls off, or falls into nearby soil. Deteriorated lead-based coatings poses a serious health risk to building occupants, particularly children. Abatement reduces or removes the health risk and includes removal, encasement, and over-coating with encapsulating coatings. Removal includes removal of the lead painted substrate or removal of the lead-based paint itself. However, removal is generally reserved for limited surface area and for surfaces where historic preservation requirements may apply. Lead paint removal techniques require high levels of control and worker protection, and also may generate significant amounts of hazardous waste.
The efficacy of a “self-healing” corrosion inhibiting coating system for use on outdoor steel cabinet enclosures for electrical equipment has been investigated. Kumar, A. and Stephenson, L. D., Accelerated Testing of Self healing Coatings, Corrosion 2003, Proceedings, National Association of Corrosion Engineers Conference, San Diego, Calif., 2003. Kumar, A and Stephenson, L. D., Self healing Coatings, Corrosion 2002, Proceedings, National Association of Corrosion Engineers Conference, Denver, Colo., 2002. U.S. patent application Ser. No. 10/377,642, Self-Healing Coating and Microcapsules to Make Same, by Kumar filed Mar. 4, 2003 and U.S. Pat. No. 7,192,993, Self-Healing Coating and Microcapsules to Make Same, to Sarangapani et al., Mar. 20, 2007, are incorporated herein by reference. Based on successful applications described therein, similar coatings were developed for over-coating wood substrates that had previously been coated with lead-based paint.
There have been efforts to effect a self-repairing capability in various materials, notably shaped articles that may be made of materials with a weakness in one or more orientations, such as cementitious materials having inherently poor tensile strength. U.S. Pat. No. 5,575,841, Cementitious Materials, to Dry, Nov. 19, 1996, and U.S. Pat. No. 5,660,624 (Aug. 26, 1997), U.S. Pat. No. 5,989,334 (Nov. 23, 1999), and U.S. Pat. No. 6,261,360 B1 (Jul. 17, 2001), each entitled Self-Repairing Reinforced Matrix Materials, all to Dry, detail a method of incorporating hollow fibers in “pourable” material to effect a self-repairing function. These inventions employ selectively releasable compounds within the hollow fiber. Because of the size of the fibers, these patents are unsuitable for repair upon a smooth surface.
Microcapsules contain minute amounts of product for specialized delivery, often size, time or location critical. They may be obtained in diameters of less than 250 microns (μ) and have been used in a variety of applications, from the pharmaceutical industry (delivery of drugs) to the textile industry (providing protective wear for HAZMAT workers). One example is U.S. Pat. No. 6,060,152, Fabric with Microencapsulated Breach Indication Coating, to Murchie, May 9, 2000. The '152 patent describes a membrane incorporating a number of different microcapsules that alert to even the smallest compromise of the fabric comprising a protective suit such as may be worn by a HAZMAT worker or health professional.
Very recent work to improve coatings by the addition of additives involves only improving the application of the coating to a substrate, not the ability of the coating to “repair” itself upon its compromise. One such example is U.S. Pat. No. 6,746,522 B2, High Molecular Weight Polymer Additive for Coating and Protective Products, to Trippe et al., Jun. 6, 2004. The '522 patent details the advantages of adding small amounts of an ultrahigh molecular weight polymer, such as polyisobutylene, to enhance coating properties of a solvent. Once, a nick compromises the coating or it is abraded, however, another separately applied application is required to protect the substrate.
Another concern in using microcapsules with solvents is the timing of delivery of the encapsulated compound. Prior patents have avoided this timing problem by mixing the microcapsules with dry powder coatings such as are used in “powder coating” applications involving elevated temperatures. U.S. Pat. No. 6,075,072, Latent Coating for Metal Surface Repair, to Guilbert et al., Jun. 13, 2000, details a self-repairing compound suitable for use in powder coating. By adding microcapsules of sufficiently small size, i.e., 10-40μ, to a dry powder form of protective coating, the resultant “self-repairing” coating is able to be powder coated upon metal substrates at suitable elevated temperatures that melt the coating to a homogenous continuous surface of approximately 200μ thickness. The microcapsules used with the '072 patent are mixed with a solvent in the dry state to prevent short-term degradation of the shells by liquid solvents.
For paint systems, self-healing coatings are fabricated by adding microcapsules containing at least one “self-healing” compound to commercially available paint primers. Paint primers may include those paints commercially termed “one coat” or “self-priming.” The microcapsules release the self-healing compound or compounds, most commonly as liquids, when the coating system is damaged. Urea formaldehyde (UF) microcapsules of 50-150 μ in diameter have been added to primers with an applied thickness of 0.1 mm (0.004″) to increase the coating service life by “self-healing” damaged areas. Verification of performance was conducted by accelerated corrosion testing on conventional coating systems using ASTM D 5894 and Electrochemical Impedance Spectroscopy (EIS). M. Kendig and J. Scully, Basic Aspects of Electrochemical Impedance Applications for the Life Prediction of Organic Coatings on Metals, Corrosion, Vol. 46, No.1, pp. 22-29, 1990. H. Hack and J. Scully, Defect Area Determination of Organic Coated Steels in Seawater Using the Breakpoint Frequency Method, J. Electrochem. Soc., Vol. 138, No. 1, pp. 33-40, 1991.
Unless appropriate materials are used to fabricate the microcapsule and its contents, it may “deploy” before the coating is applied or, upon application, spontaneously deploy improperly, i.e., without a physical compromise of the coating such as abrasion or nicking. Further, unless the microcapsule is compatible with both its contents (the encapsulated repair compound) and its surrounds (the solvent), the “application” life of the resultant mixed product may be less than desirable. These constraints have been addressed in the present invention.
Over-coating lead-based paint is one relatively inexpensive method of in situ lead hazard control. However, if over-coatings are damaged, lead dust may be exposed. A method of sealing damaged regions of the over-coating itself is needed to suppress dispersion of lead dust.